Availability of magnesium in soils

Abstract

Twenty-five soils, including some subsoils, with widely differing properties were cropped with perennial ryegrass in the glasshouse, and measures of Mg availability in the soils were related to the Mg concentration in the plants.No single measure satisfactorily characterized Mg availability. The ion activity ratio, V°M8/V°oa+Mg> when the soil neither gained nor lost Mg on equilibration with 0-02 N-CaCl 2 , was more highly correlated (r = 0-75***) with Mg concentration in the ryegrass than was the best of the capacity measures tested (CaCl 2 -extractable Mg, r = 0'68***), particularly if the potential buffering capacity (PBCjig) of the soils was also taken into account, in which case r -0-83***.Exchangeable Mg (r = 0-67***) was a poorer index of Mg availability than percentage Mg saturation (r = 0-73***) or exchangeable Mg expressed as a percentage of the total exchangeable bases (K, Na, Ca and Mg) in the soil (r = 0-81***). This latter quantity was better than the activity ratio as a predictor of Mg availability (r = 0-81*** compared with r = 0-75***).A significant proportion of the variation in Mg concentration in the ryegrass could be attributed to the K content of the soils, and closer correlations were obtained when K was included in the composite activity ratio, V a Mg/(V a ca+Mg + J3-°K)> (r = 0-88***) or when a term for exchangeable K was included in regression analysis with VoMg/Aa+Mg and P B C W (' = 0-93***).The concentration of Mg in equilibrium soil solutions was generally a less satisfactory indicator of Mg availability than were the activity ratios.Soils derived from basaltic parent material had much higher contents of readily exchangeable Mg than the remainder: parent materials other than basalt had little influence on the availability of Mg in the soils. Exchangeable Mg and per cent Mg saturation were higher in gleyed soils than in freely drained soils derived from similar parent material.

Survey of magnesium content of soils and pastures and incidence of grass tetany in three selected areas of Taranaki

“…In common with findings of previous workers (Alston 1972;Reith 1963), soil exchangeable Mg contents gave a significant and positive correlation with the Mg contents of ryegrass or mixed herbage, but generally accounted for only a small amount of the total variation in these values. Somewhat better correlations were obtained by McIntosh et al (1973) and McNaught et al (1973b), but these studies involved only one soil type.…”

“…J n this present study, the success achieved using the ratio index with mixed herbage (r = 0.73) and ryegrass (r = 0.79) over all sites in a field survey was almost equal to the correlations obtained in a glasshouse experiment, under much more controlled conditions, using cation activity ratios in equilibrium soil solutions (Alston 1972).…”

“…Interactive effects of K, Ca, and soil pH with plant Mg uptake have also been found in several earlier studies (Welte & Werner 1963;Salmon 1964;Christenson et al 1973), and various other workers have attempted to synthesise composite indices of several relevant soil properties. These attempts to improve predictions of plant Mg availability using soil data have led to the use of parameters such as percentage Mg saturation of cation exchange capacity or of total exchangeable bases (Alston 1972) and cation activity ratios of Mg, Ca, K, and H in equilibrium soil solution extracts (Bould 1964;Salmon 1964;Arnold 1967;Alston 1972). The use of composite indices often improves correlations with plant Mg values over a wide range of soil types.…”

“…Contents of exchangeable soil Mg, K, and Ca were determined by shaking 2 g of soil for 1 h with 50 ml IN (N4HOAc, pH 7 (Alston 1972). Mg, Ca, and K contents were determined on the filtered extracts using atomic absorption procedures (Perkin Elmer 306).…”

The Magnesium Status of Some Fiji Soils

“…When the data in Table 2 are converted to soil solution concentrations, values of soluble Mg range from 0.1 to 4.8 mmol/L. A few soil solution Mg values reported in the literature indicate values from less than 0.2 to 5.8 mmol/L (Alston 1972;Barber 1984). Values obtained from the Fiji soils, therefore, lie in the same range as those found elsewhere.…”